High gas permeability: at room temperature (25 °C) the permeability of silicone rubber for gases like oxygen is approximately 400 times that of butyl rubber, making silicone useful for medical applications (though precluding it from applications where gas-tight seals are necessary).

History

Technical details

More precisely called polymerizedsiloxanes or polysiloxanes, silicones are mixed inorganic-organicpolymers with the chemical formula [R2SiO]n, where R is an organic group such as methyl, ethyl, or phenyl. These materials consist of an inorganic silicon-oxygen backbone (…-Si-O-Si-O-Si-O-…) with organic side groups attached to the silicon atoms, which are four-coordinate.

In some cases organic side groups can be used to link two or more of these -Si-O- backbones together. By varying the -Si-O- chain lengths, side groups, and crosslinking, silicones can be synthesized with a wide variety of properties and compositions. They can vary in consistency from liquid to gel to rubber to hard plastic. The most common siloxane is linear polydimethylsiloxane (PDMS), a silicone oil. The second largest group of silicone materials is based on silicone resins, which are formed by branched and cage-like oligosiloxanes.

During polymerization, this reaction evolves hazardous hydrogen chloride gas. For medical uses, a process was developed where the chlorine atoms in the silane precursor were replaced with acetate groups, so that the reaction product of the final curing process is nontoxic acetic acid (vinegar). As a side effect, the curing process is also much slower in this case. This is the chemistry used in many consumer applications, such as silicone caulk and adhesives.

Silane precursors with more acid-forming groups and fewer methyl groups, such as methyltrichlorosilane, can be used to introduce branches or cross-links in the polymer chain. Ideally, each molecule of such a compound becomes a branch point. This can be used to produce hard silicone resins. Similarly, precursors with three methyl groups can be used to limit molecular weight, since each such molecule has only one reactive site and so forms the end of a siloxane chain.

Modern silicone resins are made with tetraethoxysilane, which reacts in a more mild and controllable manner than chlorosilanes.

Chemical terminology

Silicone is often mistakenly referred to as "silicon." Although silicones contain silicon atoms, they are not made up exclusively of silicon, and have completely different physical characteristics from elemental silicon.

F. S. Kipping coined the word "silicone" in 1901 to describe polydiphenylsiloxane by analogy of its formula, Ph2SiO, with the formula of the ketone benzophenone, Ph2CO (Ph stands for phenyl, C6H5). Kipping was well aware that polydiphenylsiloxane is polymeric whereas benzophenone is monomeric and noted that Ph2SiO and Ph2CO had very different chemistry.[2][3]

A true silicone group with a double bond between oxygen and silicon (see figure) does not commonly exist in nature; chemists find that the silicon atom much prefers a single bond with each of two oxygen atoms, rather than a double bond to a single atom. Polysiloxanes are still more commonly known as "silicones".

Uses

Aquarium joints

Glass aquarium manufacturers have used 100% silicone sealant exclusively from its inception in order to join glass plates, making aquariums of every size and shape. Glass joints made with silicone sealant can withstand a great deal of pressure, making obsolete the original aquarium construction method using angle-iron and putty. This same silicone is also used to make hinges in aquarium lids or even for minor repairs. Not all commercial silicones are safe for aquarium manufacture, nor is silicone used for the manufacture of acrylic aquariums as silicones do not have long-term adhesion to plastics.[8]

Automotive

In the automotive field, silicone grease is typically used as a lubricant for brake components since it is stable at high temperatures, is not water-soluble and is far less likely than other lubricants to foul.

Automotive spark plug wires are often insulated by multiple layers of silicone to prevent sparks from jumping to adjacent wires, causing misfires.

Silicone tubing is sometimes used in automotive intake systems (especially for engines with forced-induction).

Coatings

Fabrics may be coated or impregnated with silicone to form a strong, waterproof composite such as silnylon.

Cookware

As a low taint, non-toxic material, silicone can be used where contact with food is required. Silicone is becoming an important product in the cookware industry, particularly bakeware and kitchen utensils.

It is used as an insulator in heat resistant potholders and similar, however it is more conductive of heat than the less dense fiber-based ones. Silicone oven mitts are able to withstand temperatures up to 675 °F (357 °C), and allow reaching into boiling water.[9]

The solvent degrades into silica and trace amounts of water and CO2, and while the silica waste produced is hazardous if inhaled,[10] there is not currently any reason to think it is any environmentally worse than the silica present in regular beach sand. This significantly reduces the environmental impact of a typically high-polluting industry.

Additionally, liquid silicone is chemically inert, meaning it does not react with fabrics or dyes during the cleaning process. This reduces the amount of fading and shrinking that most dry-cleaned garments experience.

Electronics

Electronic components are sometimes encased in silicone to increase stability against mechanical and electrical shock, radiation and vibration. This is often called "potting".

Silicones are used where durability and high performance are demanded of components under hard conditions, such as in space (satellite technology). They are selected over polyurethane or epoxy encapsulation when a wide operating temperature range is required (−65 to 315 °C). Silicones also have the advantage of little exothermic heat rise during cure, low toxicity, good electrical properties and high purity.

The use of silicones in electronics is not without problems, however. Silicones are relatively expensive and can be attacked by solvents.[11] Silicone easily migrates as either a liquid or vapor onto other components.

Silicone contamination of electrical switch contacts can lead to failures by causing an increase in contact resistance, often late in the life of the contact, well after any testing is completed.[12][13] Use of silicone-based spray products in electronic devices during maintenance or repairs can cause later failures.

Firestops

Silicone foams have been used in North American buildings in an attempt to firestop openings within fire-resistance-rated wall and floor assemblies to prevent the spread of flames and smoke from one room to another.

When properly installed, silicone-foam firestops can be fabricated for building code compliance. Advantages include flexibility and high dielectric strength. Disadvantages include combustibility (hard to extinguish) and significant smoke development.

Silicone can also be found in aircraft technology.

Lubricants

Silicone greases are used for many purposes, such as bicycle chains. A dry-set lubricant is delivered with a solvent carrier to penetrate the chain. The solvent evaporates, leaving a clear film that lubricates but does not attract dirt and grit as much as a traditional "wet" lubricant.

Silicone personal lubricants are also available, for use in medical procedures or sexual activity.

Medicine

Silicone, particularly the gel form, is used in bandages and dressings, in breast implants and a variety of other medical uses.

Moldmaking

Two-part silicone systems are used to create rubber molds which can be used for production casting of resins, foams, rubber and low-temp alloys.

A mold made of silicone generally requires little or no mold release or surface preparation as most materials do not adhere to moldmaking silicone.

For experimental uses, ordinary one-part silicone can also be used, either to make molds, or to mold into shapes. Common vegetable cooking oils and petroleum jelly can be used on mating surfaces as a mold release agent.[14]

Personal care

Silicones are ingredients in many hair conditioner, shampoo, and hair gel products. Some silicones, notably the amine functionalized amodimethicones, are excellent conditioners. They improve combability, feel, and softness, and also lessen frizz. Another silicone family, the phenyltrimethicones, are used in reflection-enhancing and color-correcting hair products, where they increase shine and glossiness (and possibly effect subtle color changes). Phenyltrimethicones, unlike the conditioning amodimethicones, have refractive indices (typically 1.46) close to that of human hair (1.54). It should be noted that achieving both high-shine and excellent conditioning in one hair care product is much more difficult than simply adding two different silicones to the formulation, because amodimethicone and phenyltrimethicone interact with and dilute each other. This is one reason why modern hair care products, and cosmetics generally, are among the most highly engineered consumer products.

Specific grades of silicone rubber are used widely in the production of baby bottle teats due to their cleanliness, aesthetic appearance, and low extractable content.

Plumbing and building construction

The strength and reliability of silicone rubber is widely acknowledged in the construction industry.

One-part silicone sealants and caulks are in common use to seal gaps, joints and crevices in buildings. One-part silicones cure by absorbing atmospheric moisture, which helps in the professional installation.

In plumbing, silicone grease is typically applied to O-rings in faucets and valves. Whilst the film is extant it prevents lime from sticking to the brasswork.

Toys

Silicone balls have become a juggler's favorite due to the high bounce back, and are used as a response system in many low response yo-yos.[18] Silicone has the potential of replacing plastic in creating many forms of toys.

Versatile applications

Silicone caulk can be used as a basic sealant against water and air penetration.

Silicon dioxide (silica) used in the manufacture of all silicones. This is what remains when one burns silicone; burning silicone caulking or foam produces silica (as well as char) as a white powder – silica fume.